Defects in tissues resulting from disease or trauma have previously been healed through the regenerative process of wound healing. However, incomplete repair of the tissue may result when the defect is large, thereby resulting in fibrous scarring of the tissue. The fibrous scarring often possesses physical and mechanical properties that are inferior to that of non-scarred tissue.
Dramatic advances in the fields of biochemistry, cell and molecular biology, genetics, medicine, biomedical engineering and materials science have given rise to the cross-disciplinary field of tissue engineering, which uses synthetic or naturally derived, engineered scaffold/cell or scaffold/neotissue constructs for tissue regeneration. Ideally, tissue engineering aims to develop biological substitutes to solve the problem of organ and tissue deficiencies and provide medical implants. Bioreactors have been used to engineer cells and tissues.
In order to achieve optimal results in cell and tissue culture, the bioreactors should ideally operate under conditions that are as close as possible to in vivo conditions. Difficulties have arisen with known bioreactors in that they have not provided a constant and regulatory supply of nutrition and removal of metabolic byproducts. Accordingly, it is desirable that bioreactor systems maintain an organotypic environment to maintain cellular differentiation and optimal function.
The multiplication of cells is most commonly performed in culture dishes with a static medium supplemented with growth serum. Although cells grown in culture dishes multiply quite well, they tend to loose their differentiation status and are therefore functionally different from naturally grown cells. This has been found to be the case with chondrocytes from cartilage. Isolated chondrocytes flatten and look more like fibroblastic mesenchymal/stromal cells. No basic cartilage extracellular matrix results.
Known cell and tissue cultures for cell and tissue repair have utilized mono-layers of cell and tissue. For example, in a skin defect reaching a lower layer of the dermis has been treated by debriding a slough or an abnormal granulation tissue, reconstructing a normal granulation tissue by covering the defect with an allogenic skin, wound dressings or the like, and then reconstructing skin by autologous split-thickness skin grafting. A disadvantage with this procedure is that skin is taken from non-defect area of the patient's skin and some scarring may remain at the graft site. Furthermore, in circumstances where a wound extends over a wide area, it is difficult to carry out autologous split-thickness skin grafting. To prevent or diminish scarring and to increase the healing time of damaged tissues, a regenerative process has been carried out in vitro by growing cell or tissue cultures on monolayers (ie two-dimensional cell or tissue cultures) on an artificial substrate that is bathed in nutrient medium. The nature of the substrate on which the monolayers grow may be solid, such as plastic, or semisolid gels, such as collagen or agar. Disposable plastics substrates are presently used in cell or tissue culture.
Although the growth of cells in two dimensions is suitable for studying cells in culture, it lacks the cell-cell and cell-matrix interactions that are characteristic of whole tissue in vivo. To grow cells that have the cell-cell and cell-matrix interactions that are characteristic of whole tissue in vivo, the cells should preferably be grown in three-dimensions. However, the growth of three-dimensional cells requires both physical and chemical signaling. Chemical signaling is generally realized through the constituents of the culture media. Physical signaling to grow cell or tissue cultures requires the use of bioreactors to grow the cell or tissue cultures in the substrates.
Current bioreactors for growing cell tissues are designed with only a single axis of rotation. These single axis rotating bioreactors subject the growing cells on a porous substrate to only a single force vector, thereby providing physical signaling only in the direction of that single force vector. Accordingly, the cells tend not to penetrate throughout the structure of the porous substrate and growth of three-dimensional cell or tissue cultures is inhibited.
Another disadvantage with some bioreactors for growing cell tissues is that they are designed to operate in batch or semi-batch mode.
It is an object of the invention to provide a bioreactor, a system or a method for growing cell or tissue cultures that overcomes or ameliorates at least one of the disadvantages mentioned above.
A further object of the invention is to provide a bioreactor, a system or a method for growing cell or tissue cultures in vitro, that at least partially provide physical signaling in more than one force vector or flow vector or both.
A further object of the invention is to provide a bioreactor, a system or a method for growing three-dimensional cell or tissue cultures in vitro.